Leviathan (Hebrew for "Twisted; coiled") was a Biblical sea monster referred to in the Old Testament (Psalm 74:13-14; Job 41; Isaiah 27:1). The word leviathan has become synonymous with any large sea monster or creature. In the novel Moby-Dick it refers to great whales, and in Modern Hebrew, it means simply "whale".

Judaism

The Leviathan was a monstrous fish created on the fifth day of Creation. The Leviathan will be slain and its flesh served as a feast to the righteous in [the] Time to Come, and its skin used to cover the tent where the banquet will take place."
There is another religious hymn recited on the festival of Shavuot (celebrating the giving of the Torah), known as Akdamut, wherein it says: "...The sport with the Leviathan and the ox (Behemoth)...When they will interlock with one another and engage in combat, with his horns the Behemoth will gore with strength, the fish [Leviathan] will leap to meet him with his fins, with power. Their Creator will approach them with his mighty sword [and slay them both]." Thus, "from the beautiful skin of the Leviathan, God will construct canopies to shelter the righteous, who will eat the meat of the Behemoth [ox] and the Leviathan amid great joy and merriment, at a huge banquet that will be given for them." Some rabbinical commentators say these accounts are allegorical (Artscroll siddur, p. 719), or symbolic of the end of conflict.
In a legend recorded in the Midrash called Pirke de-Rabbi Eliezer it is stated that the whale which swallowed Jonah narrowly avoided being eaten by the Leviathan, which generally eats one whale each day. In a hymn by Kalir, the Leviathan is a serpent that surrounds the earth and has its tail in its mouth, like the Greek Ouroboros and the Nordic Midgard Serpent.
Legend has it that in the banquet after the end of conflict, the carcass of the leviathan will be served as a meal, along with the behemoth and the ziz.
Leviathan may also be interpreted as the sea itself, with its counterparts behemoth being the land and ziz being the air and space. Some scholars have interpreted Leviathan, and other references to the sea in the Old Testament, as highly metaphorical references to seafaring marauders who once terrorized the Kingdom of Israel. Others liken the mention to Tiamat and other similar monsters who represented the sea as a foe to the gods in myths of nearby cultures.
The Biblical references to Leviathan appear to have evolved from the Canaanite Baal cycle involving a confrontation between Hadad (Baal) and a seven headed sea monster named Lotan. Lotan is the Ugaritic orthograph for Hebrew Leviathan. Hadad defeats him. Biblical references also resemble the Babylonian creation epic Enûma Elish in which the storm god Marduk slays his mother, the sea monster and goddess of chaos and creation Tiamat and creates the earth and sky from the two halves of her corpse.

Leviathan in rabbinic literature

Creation of Leviathan According to a midrash, the leviathan was created on the fifth day (Yalkut, Gen. 12). Originally God produced a male and a female leviathan, but lest in multiplying the species should destroy the world, He slew the female, reserving her flesh for the banquet that will be given to the righteous on the advent of the Messiah (B. B. 74a).

Size The enormous size of the leviathan is thus illustrated by R. Johanan, from whom proceeded nearly all the haggadot concerning this monster: "Once we went in a ship and saw a fish which put his head out of the water. He had horns upon which was written: 'I am one of the meanest creatures that inhabit the sea. I am three hundred miles in length, and enter this day into the jaws of the leviathan'" (B. B. l.c.). When the leviathan is hungry, reports R. Dimi in the name of R. Johanan, he sends forth from his mouth a heat so great as to make all the waters of the deep boil, and if he would put his head into paradise no living creature could endure the odor of him (ib.). His abode is the Mediterranean Sea; and the waters of the Jordan fall into his mouth (Bek. 55b; B. B. l.c.).
The body of the leviathan, especially his eyes, possesses great illuminating power. This was the opinion of R. Eliezer, who, in the course of a voyage in company with R. Joshua, explained to the latter, when frightened by the sudden appearance of a brilliant light, that it probably proceeded from the eyes of the leviathan. He referred his companion to the words of Job xli. 18: "By his neesings a light doth shine, and his eyes are like the eyelids of the morning" (B. B. l.c.). However, in spite of his supernatural strength, the leviathan is afraid of a small worm called "kilbit", which clings to the gills of large fishes and kills them (Shab. 77b).
In a legend recorded in a Midrash called Pirke de-Rabbi Eliezer it is stated that the whale which swallowed Jonah narrowly avoided being eaten by the Leviathan, which generally eats one whale each day. Legend has it that in the banquet after the end of conflict, the carcass of the leviathan will be served as a meal, along with the behemoth and the ziz.